The couple - who confirmed their engagement last week after Angelina was spotted wearing a diamond ring - reportedly kept it secret for months because they wanted the press surrounding her directorial debut, 'In the Land of Blood and Honey' to die down first.

A source explained to Life & Style magazine: "Brad had the ring designed last year and made sure it was ready by Christmas because he wanted a romantic Christmas proposal - and he got it.

"They didn't announce their engagement sooner because Angie focused so much on promoting 'In the Land of Blood and Honey'. She was very busy with the movie and her Oscar campaign. She also worried that the engagement would take over press on her film and detract from that.

"There was some negative press on 'Blood and Honey' that bothered her a lot, and she didn't feel the timing was right. So they waited."

The pair - who met on the set of 2005 film 'Mr. and Mrs. Smith' while Brad was still married to Jennifer Aniston - have children Maddox, 10, Pax, nine, Zahara, seven, Shiloh, five and three-year-old twins Knox and Vivienne - who are all said to be thrilled about the engagement.

Brad's spokeswoman Cynthia Pett-Dante said of their marriage plans: "It is a promise for the future and their kids are very happy. There's no date set at this time."

Angelina was spotted wearing her engagement ring this week and jeweller Robert Procop - who has previously worked with the actress - revealed he designed it with her husband-to-be.

A spokesperson for Robert said: ""I can confirm that, yes, Robert Procop did indeed design an engagement ring for Angelina Jolie, designed in collaboration with Brad Pitt."